1. Field of the Invention
The present invention relates to a method for forming thin film structure and a thin film structure, an oscillation sensor, a pressure sensor, and an acceleration sensor. In particular, the invention relates to a method for forming a thin film structure including a plurality of polysilicon thin films combined to one another, a thin film structure formed by the relevant method, an oscillation sensor, a pressure sensor, and an acceleration sensor each having the relevant thin film structure in a sensing section.
2. Description of Related Art
In some oscillation sensors, acceleration sensors, or gyro sensors, a polysilicon thin film is used in a sensing section, and in such sensors, the periphery of the polysilicon thin film is held by a substrate so that the polysilicon thin film forms a three-dimensional structure. The polysilicon thin film for such application must be electrically conductive since an electric signal needs to be drawn to the outside. As a method of making the polysilicon thin film to be conductive, a method is typically used, in which an impurity is doped and diffused into the polysilicon thin film.
However, when the polysilicon thin film is doped with an impurity to be conductive, such conducting treatment induces compressive stress within the polysilicon thin film, causing troubles in various sensors. For example, a polysilicon thin film being made conductive is used as an oscillation film in oscillatory type sensors such as oscillation sensor (microphone) or gyro sensor, however, in the oscillator type sensors, internal stress in the polysilicon thin film significantly affects oscillation characteristics, and affects sensor accuracy. In particular, when compressive stress is induced in the polysilicon thin film, the oscillation film may be buckled and therefore not oscillated at all. In capacitance type sensors such as acceleration sensor (comblike acceleration sensor), when compressive stress is induced in the polysilicon thin film, the polysilicon thin film may be warped or buckled by an effect of the compressive stress, consequently change in capacitance of the sensing section occurs, which largely affects sensor accuracy.
In this way, while the polysilicon thin film to be used for sensing needs to be conductive for drawing the electric signal, when the film is doped with an impurity to be conductive, compressive stress is induced in the polysilicon thin film through heat treatment after impurity doping, and the compressive stress adversely affects sensor accuracy. Conversely, when tensile stress is intended to be kept in the polysilicon thin film, the polysilicon thin film is hardly made to be conductive. That is, there have been conflicting difficulties.
Thus, in an invention disclosed in U.S. Pat. No. 5,753,134 (JP-A-7-211709), as shown in FIG. 1, a thin film structure, in which a plurality of polysilicon thin films 13, 15, 17 and 19 deposited at the same condition and auxiliary layers 14, 16 and 18 are combined with each other, is formed on a substrate 11 via a sacrifice layer 12, and a thin film structure having small stress is designed to be achieved by combining the polysilicon thin films 13, 15, 17 and 19 with the auxiliary layers 14, 16 and 18.
However, such a thin film structure has not been able to obtain compatibility between processes of control of stress induced within the structure, and reduction in electric resistance. In the method described in U.S. Pat. No. 5,753,134 (JP-A-7-211709), an impurity is implanted after precipitation of corresponding partial layers respectively, or implanted after completion of the entire thin film structure in order to decrease the electric resistance of the thin film structure, however, optional tensile stress has not been able to be obtained in such a method. The reason for this is that since an impurity such as phosphor (P) works as a compression source, compression stress is induced in a film doped with the impurity, consequently small tensile stress can not be achieved (refer to Control of Residual Stress of Polysilicon Thin Films by Heavy Doping in Surface Micromaching; M. Orpana and A. O. Korhonen; Proceeding International Conference Solid-State Sensors & Actuators (Transducers '91), San Francisco, Calif., 1991 (IEEE, New York, 1991) pp. 957-960). Moreover, in such a thin film structure, diffusion of the impurity may not proceed by being hindered by the auxiliary layers 14, 16 and 18, or change in stress in the polysilicon thin films 13, 15, 17 and 19 may be increased in an impurity diffusion process, consequently stress has been hardly controlled.
As another method, a method as disclosed in U.S. Pat. No. 6,268,068 is proposed. In the method, as shown in FIG. 2, multilayer polysilicon thin films 22 to 27 are stacked on a substrate 21. By using a fact that a polysilicon thin film deposited by a LPCVD process may be a source of either compressive stress or tensile stress by changing a deposition condition, temperature during deposition is changed for each of the polysilicon thin films 22 to 27, thereby thin films having compressive stress and thin films having tensile stress are formed, and a thin film structure having small stress is designed to be achieved by appropriately combining the thin films with each other.
However, since the polysilicon thin film has an increased resistance value if an impurity is not thermally diffused after the impurity is doped, the thin film structure fabricated by such a method has a high resistance value, and therefore has not been able to be used for a sensing device from which an electric signal needs to be drawn. That is, in the method as in U.S. Pat. No. 6,268,068, the auxiliary layer such as natural oxide film is not present between a polysilicon thin film having tensile stress and a polysilicon thin film having compressive stress, and respective layers of the polysilicon thin films are continuously deposited. Therefore, the polysilicon films can not be subjected to conducting treatment during depositing the respective layers of the polysilicon thin films, consequently the thin film structure has not been able to be made to be conductive. Again in the method, stress in the polysilicon thin films has been hardly controlled, therefore a thin film structure having small tensile stress has been hardly obtained.
Patent literature 1: U.S. Pat. No. 5,753,134 (JP-A-7-211709)
Patent literature 2: U.S. Pat. No. 6,268,068
Non-patent literature 1: Control of Residual Stress of Polysilicon Thin Films by Heavy Doping in Surface Micromaching; M. Orpana and A. O. Korhonen; Proceeding International Conference Solid-State Sensors & Actuators (Transducers '91), San Francisco, Calif., 1991 (IEEE, New York, 1991) pp. 957-960